Raining inside garage!

[bobdabuilda]

My parents have had a problem with their garage for the last 5years where it literally rains in the hot weather. The garage is a link type between two detached houses and has a bitumen flat roof in place. The roof appears intact and the join onto the house appears sound with good flashing.
The garage was converted by the previous owners into 'a habitable room'! Now my parents are elderly and have the heating on full tilt all year round and its my guess that the condensation in their garage is due to a poorly ventilated flat roof space.... They have had several tradesmen out to assess the situation who are apparently 'baffled'.

The condensation only occurs in the hot weather and stops in the Winter. One tradesman reckoned that there was water ingress from the main house into the cavity wall where it then ran along the brick ties and worked its way into the roof space rafters to cause this problem. Personally I think this is a crock.

I have told them to get someone to rip the plasterboard roof panels off and take a good look but they say that the tradesmen have looked through small holes so far and don't think its needed.
Another guy has been this week and has found that theres a load of tightly packed rockwool which is soaking wet in the area that he looked at. He reckons that there is too much insulation up there which is tightly packed.

I have yet to go there to look at whats been going on but it sounds like this last guy has found the problem ie the rockwool and my theory of no ventilation is likely to be correct. Does this sound reasonable?

Do flatroofs usually have a ventilation system like soffit vents? Obviously the roof was designed for a garage and I suspect that the previous owners have cut corners and bodged the job up.
If I am right then I plan to rip the boards off and remove the rockwool. I will then put some vents in the soffits or fascia boards (if there are any!) above the level of some new kingspan polystyrene insulation and reboard/plaster. Does this sound reasonable?
Thanks

 

[Xenon]

You need air moving through to take moisture out. Is the plasterboard foil backed?

If the rockwool is tightly packed, it doesn't help, along with heating up, and windows closed.

Have a read here

 

[bobdabuilda]

Thanks Xenon. No the plasterboard isn't foil backed. There is no ventilation space whatsoever up there as the cavity is jampacked with sodden rockwool!
The rafters run perpendicular to the side of the house to which the garage is attached and the p/board is just nailed to the rafters with no batten spacers. There is no room around the outside of the roof to put in facia vents so should I just use the kingspan with a vapour control membrane and put some sort of chimney type vent on the flat rooftop?

 

[JohnD]

it might be something else, like rain, BUT if you have rockwool insulation, with no ventilation above, and porous (unfoiled) plasterboard below then it will get interstital condensation.

What this means is that warm, damp air can rise through the rockwool (as it is unsealed). the temperature within the rockwool will be (about) room temperature at the bottom and (about) outdoor temperature at the top. this will mean that within the rockwool there will be a layer which is at dewpoint. the moisture from the rising air will condense within the insulation.

As water vapour is lighter than air, any moisture in the room will rise and try to permeate the ceiling.

A few things that will fix it:

1) take out the rockwool and use a closed-cell insulation material like the rigid foam boards. As this is not porous, warm moist air cannot get into it and condense.

2) use foil-backed plasterboard to reduce the amount of warm moist air that can pass from the room into the void (you will never stop it completely as there are joints, and holes round light fittings) and seal the joints as best you can.

3) introduce cross-flow ventilation of the void (there MUST be a void) between the top of the insulation and the bottom of the cold roof. This ventilation must be of outdoor air, Cold outdoor air holds less moisture than warm inside air.

4) ventilation of the room will reduce the moisture load.

BTW the foam is a better insulator than rockwool so need not be as thick for equivalent performance. But I have not got the figures to hand. if you can use 100mm that will be pretty good.

 

[bobdabuilda]

it might be something else, like rain, BUT if you have rockwool insulation, with no ventilation above, and porous (unfoiled) plasterboard below then it will get interstital condensation.

What this means is that warm, damp air can rise through the rockwool (as it is unsealed). the temperature within the rockwool will be (about) room temperature at the bottom and (about) outdoor temperature at the top. this will mean that within the rockwool there will be a layer which is at dewpoint. the moisture from the rising air will condense within the insulation.

A few things that will fix it:

1) take out the rockwool and use a closed-cell insulation material like the rigid foam boards. As this is not porous, warm moist air cannot get into it and condense.

2) use foil-backed plasterboard to reduce the amount of warm moist air that can pass from the room into the void (you will never stop it completely as there are joints, and holes round light fittings

3) introduce cross-flow ventilation of the void (there MUST be a void) between the top of the insulation and the bottom of the cold roof. This ventilation must be of outdoor air, Cold outdoor air holds less moisture than warm inside air.

BTW the foam is a better insulator than rockwool so need not be as thick for equivalent performance. But I have not got the figures to hand. if you can use 100mm that will be pretty good.

Thanks John- yep its definitely from the rockwool as there are no leaks and it only occurs in the warmer months and esp hot days. The temp in the house is always around 25 degrees with them being crazy oldies!
( warm air holds more moisture therefore I presume that the dew point will be reached much quicker?)

The roof is coated in a silvery grey rubber like material on the outside which is glued down to marine ply which in turn is nailed to the rafters. How can I cross vent above the kingspan foam insulation (given that the rafters are perpendicular to the side of the house) without taking the entire roof covering off?

The only way I can see to avoid this, is by dropping internal ceiling height and creating a large vent space by cross battening perpendicular to the current rafters. Or maybe drilling a large bore circular hold through the middle of all the rafters and running a perforated drain pipe through this to vent all the voids and connect to a chimney vent? What do you think?

 

[JohnD]

as you say it is a link type, I can't see how to ventilate it without cross-battening. Even if you peppered the timbers with holes, I should think the impedance of so many holes would prevent airflow

I wonder if you could have a ventilation duct disguised as a cornice? It might be enough to batten out a ventilation channel along the edges rather than the whole ceiling? with one side open at one end, and one open at the other end to encourage airflow?

Or maybe you could use those roof ventilators that look like mushrooms?

Or splash out on a pitched roof over the top of the old one?

If the roof was due to be renewed, it would be easy to include cross-battening under the boards

maybe someone else knows a way

 

[bobdabuilda]

It might be enough to batten out a ventilation channel along the edges rather than the whole ceiling? with one side open at one end, and one open at the other end to encourage airflow?

Or maybe you could use those roof ventilators that look like mushrooms?

Wish I could, but the neighbours garage is joined onto theirs (two detached houses with the linked garages between) and the roof membrane continuous between the two. This problem doesn't happen in the neighbours garage but there looks like theres a slight gradient towards my parents house which probably doesn't help matters if theirs is of a similar internal construct. Not sure if they would appreciate my parents asking if I could nip in and rip their ceiling down to check!

A row of 12 mushrooms might be the only answer!! Good thinking!
If only the builders had run the rafters parallel to the main house it would be a doddle!

 

[bobdabuilda]

I wonder if the ones at the foot of this link would be ok? 12 in a row might look odd to the neighbours but if it solves the problem then I don't mind!

http://www.leisureshopdirect.com/Caravan/ventilation/Vents_-_miscellaneous.aspx

 

[JohnD]

I would get some from a roofing company. they need to be high enough to prevent rainsplash getting in, and some kind of collar built up compatible with the roof covering. I wouldn't know how to do it.

 

[bobdabuilda]

Yep looking at that link they don't look very high. A 6inch height as per the two brick dpc rule should be sufficient but I can't find any weblinks to anything suitable. Maybe there's no such thing?

 

[JohnD]

there is, I've seen them on industrial flat roofing.

 

[bobdabuilda]

I've seen them on a flat roof in New Zealand too but I can't find any on google. If you find some then please let me know! Ta

 

[Xenon]

You need something along these lines. They will require weathering into the existing roof covering.

Local depot of Asphaltic will keep the Anderson ones in stock, or Icopal i suppose i should now call them.